The invention relates to a subfloor structure of an airframe of an aircraft, particularly the subfloor structure of the fuselage cabin cell of a helicopter. The subfloor structure comprises longitudinal beams and crossbeams that are interconnected with each other and are connected to the floor and the outer bottom skin of the fuselage. Structural elements and reinforcements are inserted in or arranged on the beams.
In the aircraft construction industry, significant attention is given to the crash safety design of an airframe to limit damage or injury in the case of a crash or other impact against the lower fuselage. The subfloor structure within the airframe plays an important role in these crash safety designs. The subfloor structure is arranged in the airframe between the floor, which comprises at least a floor panel, and the outer or bottom skin of the fuselage and is attached to the floor and the skin. Structures such as seats are arranged on the floor panels. The purpose of the subfloor structure is to absorb a substantial portion of the kinetic energy of an impact or crash against the lower fuselage section of an airframe, as a means of improving the safety of pilots and passengers.
In aircraft, tanks are often arranged within the subfloor structure. In the case of an impact or crash, the tanks must be able to move, preferably laterally, to avoid rupturing. It is also important that the subfloor structure not rupture or damage the fuel tanks as a result of the impact. The outer skin of the lower fuselage should transmit the crash energy to the subfloor structure, if possible without rupturing the skin, even in a water crash impact for example.
Conventional subfloor structures are constructed under the assumption that the impact against the subfloor structure is represented primarily by an axial load against the substructure, i.e. the crash corresponds to a vertical impact. It is known that a columnar or tubular structural element, for example a cylindrical shell structure of a composite fiber material, provides the best peak force ratio with the highest specific energy absorption in the case of such an impact. For these reasons, tubular structural elements are often used in the construction of subfloor structures. Such tubular elements include forms having a cylindrical cross-section, but also polyhedron forms. These tubular elements are often made of metal or fiber material composites and are integrated into a beam. The subfloor structure then comprises several such beams arranged parallel to each other. Since these tubular or cylindrical elements are very sensitive to a non-axial loading, they are typically provided with so-called triggers that constitute defined specified failure points. These triggers reduce the peak forces that occur during an impact and enable a predictable, controlled failure mechanism, i.e. a controlled direction of failure. This controlled failure mechanism enables prediction of how and when the element will deform under the effects of the load. In a subfloor structure, the crash behavior of the structure is primarily determined by its energy absorption capability and the controlled failure mechanisms of the structural elements.
German Patent Laying-Open Publication DE 37 44 349 A1, FIG. 7, shows and describes the floor of a helicopter in which sinusoidal wave shaped, plate-like structural elements are arranged. The ends of the structural elements are fixed in a frame so that they cannot deflect away from a vertically applied force. If cylindrical shaped composite elements were to be used in such a frame, then a sandwich construction with additional cover plates and additional positioning and connecting elements would be required to contain and secure the structural elements. This is not very practical for several reasons. Fixing the structural elements within a frame increases the complexity and cost of producing the subfloor structure and furthermore leads to a significant increase in weight. It is, of course, a goal in aircraft construction to have a subfloor structure that is a lightweight construction.
The published article by C. M. Kindervater of the German Aerospace Research Establishment of Stuttgart Germany, entitled xe2x80x9cCrash Resistant Composite Airframe Structures: Design Concepts and Experimental Evaluationxe2x80x9d, which was presented November 1996 at the DGLR Conference xe2x80x9cFaserverbundwerkstoffe und -bauweisen in der Luft- und Raumfahrtxe2x80x9d in Ottobrunn Germany, discloses a subfloor structure constructed of ribs, of which the ends are formed as fiber composite elements in a Y-shape. The ribs are arranged as a cruciform, whereby two Y-shaped ends of two adjoining ribs come together at a respective intersection point and form a respective column-like tetrahedron. These straight column-like tetrahedrons serve as structural elements for absorbing crash energy.
The subfloor structure according to Kindervater provides an improvement over the subfloor structure disclosed in DE 37 44 349 A1, in that additional positioning and retainer elements are not required for the structural elements. The ribs themselves are provided with reinforcements. The subfloor structure according to Kindervater does have the disadvantage, however, that the energy absorbing structural elements are formed only at the intersection points of the cruciform structure. Thus, the use of energy absorbing structural elements is severely limited. Moreover, such column-like structural elements, whether cylindrical or formed as polyhedrons, are sensitive to lateral loads.
The majority of crash events, however, result not only in axial loads, but also in lateral loads. For example, a typical crash of an aircraft is not a strictly vertical impact, but rather involves substantial forward or lateral impact forces as well. The conventional solutions do not give adequate attention to these lateral loads. As a result, the energy-absorbing structural elements that are known and used in aircraft subfloor structures are ineffective for absorbing and dissipating the shock or impact of lateral loads.
In view of the above it is an aim of the invention to provide a subfloor structure arranged in a fuselage substructure or an airframe and that improves the crash behavior of aircraft. It is a further aim to construct such a structure as a lightweight construction that, in the event of a crash, can absorb energy from non-axial loads, as well as from axial loads, and that will not damage fuel tanks arranged between the floor and the bottom skin of the fuselage. The invention further aims to avoid or overcome the disadvantages of the prior art and to achieve additional advantages, as are apparent from the present specification.
The above objects have been achieved according to the invention in a subfloor structure comprising interconnected longitudinal beams and crossbeams that each respectively have a trapezoidal cross-section. The upper or narrow edge of the trapezoid is formed by a back or a spine, from which depend two leg flanges of the trapezoid. The leg flanges extend downwardly outwardly from each other to form a broad lower base plane of the trapezoid that is opposite and parallel to the narrow spine. The broad base is open, i.e., the trapezoidal beams are hollow and the hollow space is open to the bottom. A flap of material extends along the lower edge of each leg flange outwardly away from the trapezoidal contour of the beam on a plane that is common with the base plane of the trapezoidal beam.
The longitudinal beams and crossbeams intersect and are interconnected with each other at respective intersection areas to form a gridiron or mesh or grate arrangement. Because of the trapezoidal shape of the intersecting beams, each intersection area is shaped as a truncated four-sided cone or pyramid frustum. The leg flanges of the trapezoidal beams are preferably provided with an optimal flank angle of 10xc2x0 to 25xc2x0 relative to vertical, depending on the direction of load and type of loading that is to be handled in a crash impact. The diagonally outwardly extending leg flanges of the trapezoidal beams are reinforced. The strength of the beams can also be adapted to the particular application by using a conventional sandwich construction technique with a series of layers as needed to achieve the wall thickness appropriate for the particular application.
Structural elements can be inserted between the leg flanges of the longitudinal beams and crossbeams, at various locations of the beams. The structural elements according to the invention are each preferably formed as an oblong rectangular truncated cone or pyramid (frustum) that fits inside the beams of the subfloor structure and forms a positive fit with the inner contour of the beams formed by the spine and the trapezoidal leg flanges. The subfloor structure preferably comprises a grid of interconnected trapezoidal beams, and a plurality of rectangular cone-shaped or pyramid-shaped structural elements inserted between the leg flanges of the trapezoidal beams at particular locations or substantially along the entirety of the beams. Preferably, a respective flap of material is provided at the base of each of the four walls of the a pyramid-shaped structural element, similarly to the flap provided at the base of each trapezoidal leg flange of the beams. When the structural element is inserted into a beam, the flap from a leg flange will overlap with the flap of a wall of the element. The structural element can be adhesively affixed to the beam simply by applying adhesive to these overlapping flaps. Other types of connection between the structural elements and the beams are also possible, e.g. by riveting or providing interlocking folds or tabs.
The frustum-shaped structural elements are preferably provided with a flank angle that corresponds to the flank angle of the trapezoidal leg flanges of the beams, i.e., an angle between 10xc2x0 and 25xc2x0 relative to the vertical, so that these elements fit in a form-fitting manner into the inner contour of each beam. After the cone-shaped or pyramid-shaped elements have been inserted into a beam, the open base side of the beam can then be covered with a plate, for example, that is fixed in place.
The optimal subfloor structure is embodied as an interconnected grid of longitudinal beams and crossbeams with rectangular cones or pyramids inserted between the leg flanges of the trapezoidally shaped beams. The subfloor structure according to the invention provides the advantage that both axial and non-axial or lateral loads can be absorbed during an impact. The trigger devices of conventional subfloor structures are not necessary. The direction of failure of the subfloor structure according to the invention is toward the inner space of the trapezoidal cross-section. This provides additional improvement in the safety behavior of an aircraft because the failed material will move toward this inner space and the likelihood of the failed material damaging or rupturing the fuel tanks that are embedded in the subfloor structure is greatly reduced or eliminated.
The subfloor structure according to the invention is a lightweight structure that provides excellent energy absorption capability. As a result, the structure satisfies industry demands for a lightweight construction. Furthermore, all forces not caused by an impact can be safely transmitted through the subfloor grid structure according to the invention.